Correlation, effects atomic charges

The rules observed for reactions of Na with RX can also be correlated with certain other molecular characteristics, i.e. with effective atomic charges. 

Most of the present implementations of the CPA on the ab-initio level, both for bulk and surface cases, assume a lattice occupied by atoms with equal radii of Wigner-Seitz (or muffin-tin) spheres. The effect of charge transfer which can seriously influence the alloy energetics is often neglected. Several methods were proposed to account for charge transfer effects in bulk alloys, e.g., the so-called correlated CPA , or the screened-impurity model . The application of these methods to alloy surfaces seems to be rather complicated. 

As with the inductive effect, resonance effects on ground state properties have already been included in the procedure, PEPE, for calculating partial atomic charges. This has been achieved by generating and weighting the various resonance structures of a molecule. The significance and quality of the results has been shown by correlations and calculations of physical data 47>48-52>. 

Inamoto and co-workers (97,98) introduced a new inductive parameter i (iota) based on atomic properties of X, namely the effective nuclear charge in the valence shell and the effective principal quantum number, as well as E(X) (97). They thereby established a reasonable correlation between the a-SCSs in substituted methanes and ethanes and the t. parameters for a series of substituents not including X = CN and I (97). 

In our non-BO calculations performed so far, we have considered atomic systems with only -electrons and molecular systems with only a-electrons. The atomic non-BO calculations are much less complicated than the molecular calculations. After separation of the center-of-mass motion from the Hamiltonian and placing the atom nucleus in the center of the coordinate system, the internal Hamiltonian describes the motion of light pseudoelectrons in the central field on a positive charge (the charge of the nucleus) located in the origin of the internal coordinate system. Thus the basis functions in this case have to be able to accurately describe only the electronic correlation effect and the spherically symmetric distribution of the electrons around the central positive charge. 

As effective nuclear charge (Z on the central atom increases, acid strength is likewise increased. Thus, a larger nuclear charge draws the electrons closer to the nucleus and binds them more tightly. 1 point given for correlation between and add strength. 

Johnson and Rice used an LCAO continuum orbital constructed of atomic phase-shifted coulomb functions. Such an orbital displays all of the aforementioned properties, and has only one obvious deficiency— because of large interatomic overlap, the wavefunction does not vanish at each of the nuclei of the molecule. Use of the LCAO representation of the wavefunction is equivalent to picturing the molecule as composed of individual atoms which act as independent scattering centers. However, all the overall molecular symmetry properties are accounted for, and interference effects are explicitly treated. Correlation effects appear through an assigned effective nuclear charge and corresponding quantum defects of the atomic functions. 

In the framework of perturbation theory (PT) the correlation effects can be accounted for fairly accurately. However, various versions of PT as a rule have been practically applied only to light atoms and multiply charged ions having a number of electrons IV < 10 [65, 66]. 

Multiply charged ions represent a peculiar world, essentially differing from the world of neutral atoms or their first ions. In this chapter we shall try to describe these peculiarities. Main attention will be paid to the need for a combination of theoretical and experimental studies of the spectra of highly ionized atoms, and to the role of relativistic and correlation effects for multiply charged ions, and to the regularities and irregularities of the behaviour of their spectral characteristics along isoelectronic sequences. 

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